This invention relates to a plunger lift for moving liquids upwardly in a hydrocarbon well and more particularly to an improved approach for separating the components of a multipart piston at the surface and an improved approach for cycling the piston.
There are many different techniques for artificially lifting formation liquids from hydrocarbon wells. Reciprocating sucker rod pumps are the most commonly used because they are the most cost effective, all things considered, over a wide variety of applications. Other types of artificial lift include electrically driven down hole pumps, hydraulic pumps, rotating rod pumps, free pistons or plunger lifts and several varieties of gas lift. These alternate types of artificial lift are more cost effective than sucker rod pumps in the niches or applications where they have become popular. One of these alternative types of artificial lift is known as a plunger lift, which is basically a free piston that moves upwardly in the well to move formation liquids to the surface. Typically, plunger lifts are used in gas wells that are loading up with formation liquids thereby reducing the amount of gas flow.
Gas wells reach their economic limit for a variety of reasons. A very common reason is the gas production declines to a point where the formation liquids are not readily moved up the production string to the surface. Two phase upward flow in a well is a complicated affair and most engineering equations thought to predict flow are only rough estimates of what is actually occurring. One reason is the changing relation of the liquid and of the gas flowing upwardly in the well. At times of more-or-less constant flow, the liquid acts as an upwardly moving film on the inside of the flow string while the gas flows in a central path on the inside of the liquid film. The gas flows much faster than the liquid film. When the volume of gas flow slows down below some critical values, or stops, the liquid runs down the inside of the flow string and accumulates in the bottom of the well.
If sufficient liquid accumulates in the bottom of the well, the well is no longer able to flow because the pressure in the reservoir is not able to start flowing against the pressure of the liquid column. The well is said to have loaded up and died. Years ago, gas wells were plugged much quicker than today because it was not economic to artificially lift small quantities of liquid from a gas well. At relatively high gas prices, it is economic to keep old gas wells on production. It has gradually been realized that gas wells have a life cycle that includes an old age segment where a variety of techniques are used to keep liquids flowing upwardly in the well and thereby prevent the well from loading up and dying.
There are many techniques for keeping old gas wells flowing and the appropriate one depends on where the well is in its life cycle. For example, the first technique is to drop soap sticks into the well. The soap sticks and some agitation cause the liquids to foam. The well is then turned to the atmosphere and a great deal of foamed liquid is discharged from the well. Later in its life cycle, when soaping the well has become much less effective, a string of 1xe2x80x3 or 1xc2xdxe2x80x3 tubing is run inside the production string. The idea is that the upward velocity in the small tubing string is much higher which keeps the liquid moving upwardly in the well to the surface. A rule of thumb is that wells producing enough gas to have an upward velocity in excess of 10xe2x80x2/second will stay unloaded. Wells where the upward velocity is less than 5xe2x80x2/second will always load up and die.
As some stage in the life of a gas well, these techniques no longer work and the only approach left to keep the well on production is to artificially lift the liquid with a pump of some description. The logical and time tested technique is to pump the accumulated liquid up to the tubing string with a sucker rod pump and allow produced gas to flow up the annulus between the tubing string and the casing string. This is normally not practical in a 2xe2x85x9exe2x80x3 tubingless completion unless one tries to use hollow rods and pump up the rods, which normally doesn""t work very well or very long. Even then, it is not long before the rods cut a hole in the 2xe2x85x9exe2x80x3 string and the well is lost. In addition, sucker rod pumps require a large initial capital outlay and either require electrical service or elaborate equipment to restart the engine.
Free pistons or plunger lifts are another common type of artificial pumping system to raise liquid from a well that produces a substantial quantity of gas. Conventional plunger lift systems comprise a piston that is dropped into the well by stopping upward flow in the well, as by closing the wing valve on the well head. The piston is often called a free piston because it is not attached to a sucker rod string or other mechanism to pull the piston to the surface. When the piston reaches the bottom of the well, it falls into the liquid in the bottom of the well and ultimately into contact with a bumper spring, normally seated in a collar or resting on a collar stop. The wing valve is opened and gas flowing into the well pushes the piston upwardly toward the surface, pushing liquid on top of the piston to the surface. Although plunger lifts are commonly used devices, there is as much art as science to their operation.
A major disadvantage of conventional plunger lifts is the well must be shut in so the piston is able to fall to the bottom of the well. Because wells in need of artificial lifting are susceptible to being easily killed, stopping flow in the well has a number of serious effects. Most importantly, the liquid on the inside of the production string falls to the bottom of the well, or is pushed downwardly by the falling piston. This is the last thing that is desired because it is the reason that wells load up and die. In response to the desire to keep the well flowing when a plunger lift piston is dropped into the well, attempts have been made to provide valved bypasses through the piston which open and close at appropriate times. Such devices are to date quite intricate and these attempts have so far failed to gain wide acceptance.
A more recent development is of multi-part plungers which may be dropped into a well while formation contents are flowing upwardly in the well as shown in U.S. Pat. Nos. 6,148,923, 6,209,637 and 6,467,541.
In this invention, an improved technique is used to hold part of a multipart piston at the surface. Flow of formation contents is directed upwardly around and/or through a sleeve comprising part of the piston to produce a pressure drop across the sleeve sufficient to hold the sleeve in the wellhead and offset gravity. The sleeve is released by momentarily interrupting flow from the well, as by the use of a motorized wing valve on the well head. As soon as flow is interrupted, the pressure drop across the sleeve disappears and the sleeve falls into the well.
In another aspect of this invention, a sensor is used to detect liquid flow, as opposed to gas flow and a parameter or value is obtained that is proportional to the amount of liquid being ejected from the well by the free piston. If the amount of liquid is smaller than desired, part of the multipart piston is retained in the well head a little longer time than previously. If the amount of liquid is larger than desired, part of the multipart piston is retained in the well head a little shorter time than previously. It is desired to retrieve a small quantity of liquid on each trip of the free piston, typically on the order of xe2x85x9 to xc2xd barrel per trip.
It is an object of this invention to provide an improved plunger lift.
A more specific object of this invention is to provide an improved separator for a multipart piston of a plunger lift that acts to hold a plunger sleeve and release it without moving parts acting on the plunger.
A further object of this invention is to provide an improved technique for determining when to drop the sleeve of a multipart piston.
These and other objects of this invention will become more fully apparent as this description proceeds, reference being made to the accompanying drawings and appended claims.